1. Field of the Invention
The present invention relates to an image recording material that can be used as an offset printing master and, in particular, to a positive-working lithographic printing starting plate for an infrared laser for so-called direct platemaking, in which platemaking can be carried out directly from a digital signal sent from a computer, etc.
2. Description of the Related Art
The development of lasers in recent years has been remarkable and, in particular, with regard to solid-state lasers and semiconductor lasers having emission regions from near-infrared to infrared, compact devices having high output are readily available. These lasers are very useful as exposure light sources when carrying out direct platemaking by means of scanning exposure based on a digital data signal sent from a computer, etc.
A lithographic printing material is a material used for making a lithographic printing starting plate having a heat-sensitive layer or a light-sensitive layer. With regard to a positive-working lithographic printing plate material for an infrared laser, an aqueous alkaline solution-soluble binder resin, an infrared-absorbing dye (hereinafter also called an ‘IR dye’) that absorbs infrared light and generates heat, etc. are essential components; in unexposed areas (image areas) the IR dye, etc. functions as a dissolution inhibitor that substantially degrades the solubility of the binder resin by interacting with the binder resin, and in exposed areas (non-image areas) heat generated therein weakens the interaction between the IR dye, etc. and the binder resin, they dissolve in an alkaline developer, and a hydrophilic surface of a support is exposed, thereby forming a lithographic printing plate.
However, in such a positive-working lithographic printing plate material for an infrared laser in which unexposed areas (image areas) are receptive to a printing ink, it cannot be said that there is sufficient difference between the dissolution resistance to the developer in unexposed areas (image areas) and the solubility in exposed areas (non-image areas) under various conditions of use, and there is the problem that overdevelopment and underdevelopment easily occur due to variations in the conditions of use. Furthermore, fine scratches are formed by, for example, touching the surface during handling, the surface state is thus easily changed, and when such fine scratches or a slight change in the surface state occurs, the solubility increases, and the unexposed areas (image areas) dissolve during development, thus forming scratch marks; since image defects occur in image areas, there are the problems that degradation in plate life and incomplete laydown are caused.
Such problems are due to an intrinsic difference in platemaking mechanism between the positive-working lithographic printing plate material for an infrared laser and a positive-working lithographic printing plate material that is processed by UV exposure. That is, the positive-working lithographic printing plate material processed by UV exposure contains as essential components an aqueous alkaline solution-soluble binder resin and an onium salt or a quinone diazide compound, and this onium salt or quinone diazide compound not only functions as a dissolution inhibitor by interaction with the binder resin in unexposed areas (image areas) but also functions as a dissolution promoter in exposed areas (non-image areas) by decomposing under light and generating an acid.
In contrast, the IR dye, etc. in the positive-working lithographic printing plate material for an infrared laser functions only as a dissolution inhibitor for unexposed areas (image areas) and does not promote dissolution in exposed areas (non-image areas). Therefore, in order to enhance the difference in solubility between unexposed and exposed areas of the positive-working lithographic printing plate material for an infrared laser, a binder resin having high solubility in an alkaline developer must be used, and there are the problems that the scratch resistance is poor and the state prior to development is unstable.
As a method for increasing the difference in solubility between exposed and unexposed areas, JP-A-10-282643 (JP-A denotes a Japanese unexamined patent application publication) discloses that in alkali development, the proportion of film remaining in unexposed areas is improved by adding an organic acid to a positive type light-sensitive composition containing a light-heat converting substance and an alkali-soluble resin. Furthermore, JP-A-2001-324808 discloses a positive type light-sensitive composition containing a light-heat converting substance and a specific mass ratio of a novolac resin and an acrylic resin, thereby giving a system having a high sensitivity and an improved proportion of film remaining in unexposed areas. Furthermore, JP-A-11-143076, JP-A-11-190903, and the like disclose that a combination of an alkali-soluble resin having phenolic hydroxyl groups and an acidic color-developing dye or a basic color-developing dye enables the proportion of film remaining in the unexposed areas and the contrast between unexposed and exposed areas to be improved.
However, even in the techniques disclosed above, the difference in alkali solubility between exposed and unexposed areas (the so-called dissolution discrimination) is not large, resulting in insufficient development latitude. In addition, further improvement in sensitivity is also desired in order to heighten productivity in the platemaking of the lithographic printing plate.
As a method for improving the above-mentioned problems, a heat-sensitive layer having a multilayer structure has been proposed (ref. JP-A-10-250255). However, in this method a change in solubility is in practice not used in a key process of image formation; instead, it employs a function of changing the penetration of a developer by abrasion of an upper layer, and various problems are caused due to the abrasion. Moreover, there is a known recording layer in which a novolac resin is used in an uppermost layer and a lower layer, an image being formed by utilizing variation in solubility in a silicate-containing alkaline developer, but an adequate difference in solubility between unexposed areas and exposed areas could not be obtained (ref. JP-A-11-223935). There is also known a positive-working lithographic printing starting plate having as an upper layer a heat-sensitive layer that becomes soluble in an alkali by exposure to an infrared laser and as a lower layer an alkali-soluble polymer layer, the positive-working lithographic printing plate being developed with an alkaline developer containing as main components a base and an organic compound having a buffering action, but a further enhancement of the difference in solubility between unexposed areas and exposed areas and, in particular, of the solubility of the exposed areas (the so-called dissolution promotion) is desired (ref. JP-A-2002-182400).